Vapor Pressure Calculator (Raoult's Law)
Calculate vapor pressures in ideal liquid mixtures using Raoult's Law.
Find total and partial pressures, vapor composition, and when ideal behavior applies.
Raoult’s Law describes the vapor pressure of ideal solutions — mixtures where components have similar intermolecular forces.
Partial pressure of component A:
P_A = x_A × P°_A
Total vapor pressure:
P_total = x_A × P°_A + x_B × P°_B
Where:
- x_A, x_B = mole fractions in the liquid phase (x_A + x_B = 1)
- P°_A, P°_B = vapor pressures of pure components
Vapor composition (mole fraction in vapor):
y_A = P_A / P_total = x_A × P°_A / P_total
Positive and negative deviations from Raoult’s Law:
- Ideal (Raoult’s Law): A-B interactions ≈ A-A and B-B interactions (e.g., benzene-toluene)
- Positive deviation: A-B interactions weaker than pure components (e.g., ethanol-water, acetone-hexane) → P_total > Raoult’s Law
- Negative deviation: A-B interactions stronger (e.g., acetone-chloroform, HCl-water) → P_total < Raoult’s Law
Benzene-toluene system at 80°C (approximately ideal):
- P°_benzene ≈ 760 mmHg (normal boiling point!)
- P°_toluene ≈ 290 mmHg
Applications:
- Fractional distillation (separating components by boiling point)
- Understanding azeotropes (constant-boiling mixtures that cannot be separated by simple distillation)
- Colligative properties (related: vapor pressure lowering by non-volatile solutes)
Colligative properties connection: When a non-volatile solute dissolves, x_solvent decreases → vapor pressure decreases. ΔP = x_solute × P°_solvent (Raoult’s Law for non-volatile solute — see vapor pressure lowering calculator).
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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